This invention relates to optical sensors and, in particular, to sensors based on the use of surface plasmon-polaritons (SPP) and guided modes for coupling orthogonal radiation modes.
SPP based optical sensors work on the principal of detecting changes in the SPP resonance as a thin layer on the metal which supports the SPP is changed in either dielectric constant or thickness.
In general there are two ways of monitoring SPP resonances
(a) change in reflectivity as a function of angle of incidence (momentum of the photons) PA1 (b) change in reflectivity as a function of wavelength (energy of the photons).
Typical devices use a grating coated in an opaque layer of gold or silver and the system is set so that the detector examines the edge of the resonance--the region of steepest gradient in R v. .theta. or R v. .lambda.. Any shift in the SPP resonance is then recorded as a minimum in received intensity.